Mission to a Mare Ridge (1968)

Image: NASA

The 27 January 1967 Apollo 1 fire undermined confidence in NASA’s ability to put a man on the moon by 1970. The unmanned Apollo 4 (11 November 1967) and Apollo 5 (22 January 1968) missions, respectively the successful first test of the Saturn V rocket and the successful first test of the Lunar Module (LM), did much to restore faith in the U.S. civilian space agency. Two weeks after the fire’s solemn first aniversary, M. T. Yates, an engineer with Bellcomm, NASA’s Apollo planning contractor, completed a memorandum which demonstrated that renewed confidence. In it, he proposed a surface exploration plan for the third Apollo manned moon landing mission.

In keeping with the lunar mission nomenclature proposed in Bellcomm’s January 1968 Lunar Exploration Program Plan, Yates designated the mission Lunar Landing Mission-3 (LLM-3). An “early Apollo” mission, LLM-3 would include a 35-hour stay on the moon, three three-hour moonwalks by two astronauts, and surface exploration on foot no farther than one kilometer from the LM. Critical for detailed moonwalk planning would be the LLM-3 LM’s ability to set down within a 200-meter-diameter circle centered on a pre-selected landing point. LLM-1 and LLM-2 would be counted as successful if they managed to touch down anywhere on a smooth mare (Latin for “sea”) within an ellipse with a total area of 235 square kilometers; LLM-3’s landing ellipse area would total just 0.25 square kilometers.

Yates selected as his LLM-3 landing site an area photographed by the Lunar Orbiter III spacecraft between February and October 1967. Located at 36° west, 3° south, it lay in Oceanus Procellarum directly south of the prominent ray crater Kepler. Specifically, he aimed the LLM-3 LM at a half-kilometer-wide mare ridge (image at top of post) with a fresh, 200-meter-wide crater on top. Mare ridges are common features on the dark-hued lunar maria; some mare ridges are faults, where the mare’s basaltic crust has shifted, cracked, and rumpled, while others might indicate lava movement just beneath the surface in the past. Yates expected that the crater on the mare ridge would act as a natural drill hole, enabling the astronauts to collect geologic samples from deep inside the ridge which they could never obtain otherwise.

The first moonwalk of the LLM-3 mission would see the two astronauts, designated A and B, working together to set up the Apollo Lunar Scientific Experiment Package (ALSEP) north of their LM. The LLM-3 ALSEP would include a hand-held drill for collecting subsurface core samples and heat flow probes for installation in the drill holes. The astronauts would then move south past the LM to the rim of the crater. During the second moonwalk, astronaut B would descend into the crater while astronaut A monitored his activities from its rim. In addition to keeping an eye on his colleague, A would relay radio signals from B to the LM for transmission to Earth. This would be necessary, Yates wrote, because the crater rim would block B’s radio signals.

In the third and final LLM-3 moonwalk, astronaut B would move westward down a short canyon to the mare floor, then would walk south along the ridge-mare contact. Astronaut A, meanwhile, would walk along the mare ridge crest to keep B in sight and again relay his radio signals to the LM. The astronauts would then meet up and return to the LM via the east rim of the crater.

No Apollo mission explored a mare ridge, and Yates’s proposed radio relay technique was never used. The second Apollo lunar landing mission, Apollo 12, amply demonstrated the pinpoint landing capability Yates rightly deemed crucial by setting down near the derelict Surveyor III lander in November 1969. Apollo 14, the third successful Apollo lunar landing mission, used this capability to land near Cone Crater, a naturally occurring drill hole that permitted Al Shepard and Ed Mitchell to sample deep within the Fra Mauro Formation in February 1971.